1. CS 356: Computer Network Architectures Lecture 12: Dynamic routing protocols: Link State Chapter 3.3.3 Xiaowei Yang xwy@cs.duke.edu

2. Today Routing information protocol (RIP) Link-state routing – Algorithm – Protocol: Open shortest path first (OSPF)

3. Dynamic Routing There are two parts related to IP packet handling: 1.Forwarding (per-node operation): how to pass a packet from an input interface to the output interface of a router 2.Routing (distributed algorithm): how to find and setup a route ? –Implemented by dynamic routing protocols

4. What distributed routing algorithms common routing protocols use Routing information protocol (RIP)Distance vector Interior Gateway routing protocol (IGRP, cisco proprietary) Distance vector Open shortest path first (OSPF)Link state Intermediate System-to-Intermediate System (IS-IS) Link state Border gateway protocol (BGP)Path vector Routing protocolDistributed algorithm

5. Distance vector algorithm: initialization Let D x (y) be the estimate of least cost from x to y Initialization: – Each node x knows the cost to each neighbor: c(x,v). For each neighbor v of x, D x (v) = c(x,v) – D x (y) to other nodes are initialized as infinity Each node x maintains a distance vector (DV): – D x = [D x (y): y ∈ N ]

6. Distance vector algorithm: updates Each node x sends its distance vector to its neighbors, either periodically, or triggered by a change in its DV When a node x receives a new DV estimate from a neighbor v, it updates its own DV using the B-F equation: – If c(x,v) + D v (y) < D x (y) then D x (y) = c(x,v) + D v (y) Sets the next hop to reach the destination y to the neighbor v Notify neighbors of the change The estimate D x (y) will converge to the actual least cost d x (y)

7. RIP - Routing Information Protocol A simple intradomain protocol Straightforward implementation of Distance Vector Routing Each router advertises its distance vector every 30 seconds (or whenever its routing table changes) to all of its neighbors RIP always uses 1 as link metric Maximum hop count is 15, with “16” equal to “  ” Routes are timeout (set to 16) after 3 minutes if they are not updated

8. RIP - History Late 1960s : Distance Vector protocols were used in the ARPANET Mid-1970s: XNS (Xerox Network system) routing protocol is the ancestor of RIP in IP (and Novell’s IPX RIP and Apple’s routing protocol) 1982Release of routed for BSD Unix 1988RIPv1 (RFC 1058) - classful routing 1993RIPv2 (RFC 1388) - adds subnet masks with each route entry - allows classless routing 1998Current version of RIPv2 (RFC 2453)

9. RIPv1 Packet Format One RIP message can have up to 25 route entries 1: request 2: response 2: for IP Address of destination Cost (measured in hops) 1: RIPv1

10. RIPv2 RIPv2 is an extends RIPv1: – Subnet masks are carried in the route information – Authentication of routing messages – Route information carries next-hop address – Uses IP multicasting to send routing messages Extensions of RIPv2 are carried in unused fields of RIPv1 messages

11. RIPv2 Packet Format One RIP message can have up to 25 route entries 1: request 2: response 2: for IP Address of destination Cost (measured in hops) 2: RIPv2

12. RIPv2 Packet Format Used to provide a method of separating "internal" RIP routes (routes for networks within the RIP routing domain) from "external" RIP routes Identifies a better next-hop address on the same subnet than the advertising router, if one exists (otherwise 0….0) 2: RIPv2 Subnet mask for IP address

13. RIP Messages This is the operation of RIP in routed. Dedicated port for RIP is UDP port 520. Two types of messages: – Request messages used to ask neighboring nodes for an update – Response messages contains an update

14. Routing with RIP Initialization: Send a request packet (command = 1, address family=0..0) on all interfaces: RIPv1 uses broadcast if possible, RIPv2 uses multicast address 224.0.0.9, if possible requesting routing tables from neighboring routers Request received: Routers that receive above request send their entire routing table Response received: Update the routing table Regular routing updates: Every 30 seconds, send all or part of the routing tables to every neighbor in an response message Triggered Updates: Whenever the metric for a route changes, send entire routing table.

15. RIP Security Issue: Sending bogus routing updates to a router RIPv1: No protection RIPv2: Simple authentication scheme 2: plaintext password

16. RIP Problems RIP takes a long time to stabilize – Even for a small network, it takes several minutes until the routing tables have settled after a change RIP has all the problems of distance vector algorithms, e.g., count-to-Infinity » RIP uses split horizon to avoid count-to-infinity The maximum path in RIP is 15 hops

17. Link-state routing aims to address those problems

18. Distance Vector vs. Link State Routing With distance vector routing, each node has information only about the next hop to a destination Node A: to reach F go to B Node B: to reach F go to D Node D: to reach F go to E Node E: go directly to F Distance vector routing makes poor routing decisions if directions are not completely correct (e.g., because a node is down). – Count to infinity If parts of the directions incorrect, the routing may be incorrect until the routing algorithms has re-converged. A A B B C C D D E E F F

19. 19 Distance Vector vs. Link State Routing In link state routing, each node has a complete map of the topology If a node fails, each node can calculate the new route Challenge: All nodes need to have a consistent view of the network A A B B C C D D E E F F ABC DE F ABC DE F ABC DE F ABC DE F ABC DE F ABC DE F

20. Link State Routing: Basic operations 1.Each router establishes a relationship (“adjacency”) with its neighbors 2. Each router generates a link state advertisement (LSA) which is reliably flooded to all routers ID of the node that creates the LSP List of neighbors and costs of the links A sequence number TTL 3. Each router maintains a database of all received LSAs (topological database or link state database), which describes the network has a graph with weighted edges 4. Each router uses its link state database to run a shortest path algorithm (Dijkstra’s algorithm) to produce the shortest path to each network 20

21. 21 Link state routing: graphical illustration a b cd 3 1 6 2 a 3 6 b c a b c 3 1 a b cd 1 6 cd 2 a’s view b’s view c’s view d’s view Collecting all pieces yield a complete view of the network!

22. 22 Operation of a Link State Routing protocol Received LSPs IP Routing Table Dijkstra’s Algorithm Link State Database LSPs are flooded to other interfaces

23. Reliable flooding We’ve learned a flooding algorithm used by Ethernet switches Question: why is it insufficient for link-state routing? – Lost LSAs may result in inconsistent topologies at different routers – Inconsistent topologies may lead to routing loops

24. Reliable flooding LSPs are transmitted reliably between adjacent routers – ACK and retransmission For a node x, if it receives an LSA sent by y – Stores LSA(y) if it does not have a copy – Otherwise, compares SeqNo. If newer, store; otherwise discard – If a new LSA(y), floods LSA(y) to all neighbors except the incoming neighbor

25. An example of reliable flooding

26. When to flood an LSP Triggered if a link’s state has changed – Detecting failure Neighbors exchange hello messages If not receiving hello, assume dead Periodic generating a new LSA – Fault tolerance (what if LSA in memory is corrupted?

27. Path computation Dijkstra’s Shortest Path Algorithm for a Graph Input: Graph (N,E) with N the set of nodes and E the set of edges c vw link cost (c vw = ∞ if (v,w)  E, c vv = 0) s source node. Output : D n cost of the least-cost path from node s to node n M = {s}; for each n  M D n = c sn ; while (M  all nodes) do Find w  M for which D w = min{D j ; j  M}; Add w to M; for each neighbor n of w and n  M D n = min[ D n, D w + c wn ]; Update route; enddo

28. Practical Implementation: forward search algorithm More efficient: extracting min from a smaller set rather than the entire graph Two lists: Tentative and Confirmed Each entry: (destination, cost, nextHop) 1.Confirmed = {(s,0,s)} 2.Let Next = Confirmed.last 3.For each Nbr of Next – Cost = my  Next + Next  Nbr If Neighbor not in Confirmed or Tentative – Add (Nbr, Cost, my.Nexthop(Next)) to Tentative – If Nbr is in Tentative, and Cost is less than Nbr.Cost, update Nbr.Cost to Cost 4.If Tentative not empty, pick the entry with smallest cost in Tentative and move it to Confirmed, and return to Step 2 – Pick the smallest cost from a smaller list Tentative, rather than the rest of the graph

29. StepConfirmedTentative 1(D,0,-) 2 3 4 5 6 7

30. StepConfirmedTentative 1(D,0,-) 2 (B,11,B), (C,2,C) 3(D,0,-), (C,2,C)(B,11,B) 4(D,0,-), (C,2,C)(B,5,C) (A,12,C) 5(D,0,-), (C,2,C), (B,5,C)(A,12,C) 6(D,0,-),(C,2,C),(B,5,C)(A,10,C) 7(D,0,-),(C,2,C),(B,5,C), (A,10,C)

31. OSPF OSPF = Open Shortest Path First – Open stands for open, non-proprietary A link state routing protocol The complexity of OSPF is significant – RIP (RFC 2453 ~ 40 pages) – OSPF (RFC 2328 ~ 250 pages) History: – 1989: RFC 1131 OSPF Version 1 – 1991: RFC1247 OSPF Version 2 – 1994: RFC 1583 OSPF Version 2 (revised) – 1997: RFC 2178 OSPF Version 2 (revised) – 1998: RFC 2328 OSPF Version 2 (current version)

32. Features of OSPF Provides authentication of routing messages – Similar to RIP 2 Allows hierarchical routing – Divide a domain into sub-areas Enables load balancing by allowing traffic to be split evenly across routes with equal cost

33. OSPF Packet Format Destination IP: neighbor’s IP address or 224.0.0.5 (ALLSPFRouters) or 224.0.0.6 (AllDRouters) TTL: set to 1 (in most cases) OSPF packets are not carried as UDP payload! OSPF has its own IP protocol number: 89 Link state advertisement

34. OSPF Common header 2: current version is OSPF V2 Message types: 1: Hello (tests reachability) 2: Database description 3: Link Status request 4: Link state update 5: Link state acknowledgement ID of the Area from which the packet originated Standard IP checksum taken over entire packet 0: no authentication 1: Cleartext password 2: MD5 checksum (added to end packet) Authentication passwd = 1: 64 cleartext password Authentication passwd = 2: 0x0000 (16 bits) KeyID (8 bits) Length of MD5 checksum (8 bits) Nondecreasing sequence number (32 bits) Prevents replay attacks

35. OSPF LSA Format LSAs – Type 1: cost of links between routers – Type 2: networks to which the router connects – Others: hierarchical routing LSA Header Link 1 Link 2 Link State ID link state sequence number advertising router Link AgeLink Type checksumlength

36. Type 1 LSA Link state ID and Advertising router are the same, 32-bit router ID Link ID: router ID at the other end of the link Link Data: identify parallel links Metric: cost of the link Type: types of the link e.g., point-to-point

37. Open question How to set link metrics? Design choice 1: all to 1 Design choice 2: based on load – Problems? In practice: static

38. Hierarchical OSPF

39. 39 Hierarchical OSPF Two-level hierarchy: local area, backbone. – Link-state advertisements only in area – Each nodes has detailed area topology; only know direction (shortest path) to nets in other areas. Area border routers: “summarize” distances to nets in own area, advertise to other Area Border routers. Backbone routers: run OSPF routing limited to backbone.

40. Scalability and Optimal Routing A frequent tradeoff in network design Hierarchy introduces information hiding ABR

41. OSPF summary A link-state routing protocol Each node has a map of the network and uses Dijkstra to compute shortest paths Nodes use reliable flooding to keep an identical copy of the network map

42. Summary Routing information protocol (RIP) Link-state routing – Algorithm – Protocol: Open shortest path first (OSPF)